1. Field of the invention
This invention relates to a method and apparatus for determining aerosol size distributions, and in particular a method and apparatus for determining aerosol size distributions using laser beams.
2. Description of the Prior Art
Various devices have been disclosed in the prior art to determine aerosol size distributions. Often these devices employ Rayleigh-scattering techniques in which a collimated laser beam or a focused tungsten source projects light through aerosol particles. Thereafter the light is scattered by the particles and is collected by a mirror or lens and then relayed to a photocathode of a photo-multiplier. The photo-multiplier current pluses are then amplified, shaped, counted, and histogrammed by appropriate electronic circuitry. The number of counts registered per second then indicates the particle concentration, and the histogram provides a size-frequency distrbution for the detected particles (see U.S. Pat. No 4,071,298 to Falconer). There has previously been developed a method and means for measuring the total particle volume or mass by use of a monochromatic laser light beam directed through a collection of particles and then through a spacial filter which serves to establish a third power relationship between the defracted light flux from each particle transmitted by the spacial filter and the radius of the particle (see U.S. patent application Ser. No. 403,288, now U.S. Pat. No. 3,873,206, filed Oct. 3, 1973 by Wilcock).
Other prior art devices have utilized computation of distribution data by the use of digital computers in systems which laboriously examine individual particles from the collection to be analyzed and use counting procedures to obtain statistics on the aggregate collection.
Yet another metheod and apparatus for particulate monitoring, as disclosed is U.S. Pat. No. 4,015,135 to Tipton, involves illuminating a sample of a suspension having a predetermined known volume containing a particle population sufficiently small to permit substantially simultaneous back-scattered radiation measurement thereof for a given particle classification determination with a monochromatic light source, directing the essentially back-scattered radiation detector along a line substantially coincident with the optical axis of the detector such that the detector transduces the back-scattered radiation from individual particles in the sample into electrical pulse signals, and determining particle size, number and distribution as a function of signal pulse height.
Examples of other relevant particulate and aerosol size distribution monitoring devices can be found in U.S. Pat. No. 4,052,600 to Wertheimer; U.S. Pat. No. 4,037,965 to Weiss; U.S. Pat. No. 4,017,186 to Shofner et al.; U.S. Pat. No. 3,835,315 to Gravitt; U.S. Pat. No. 3,820,897 to Roess; U.S. Pat. No. 3,788,742 to Garbury; U.S. Pat. No. 3,700,333 to Charlson et al.; U.S. Pat. No. 3,646,352 to Bol et al.; and U.S. Pat. No. 3,556,659 to Hawes.
A review, however, of the prior art methods and apparata discussed above reveals that these devices are generally more concerned with particulate rather than aerosol distribution analysis, and often employ highly complex computational techniques requiring sophistacted software and hardware to process the large volumes of data thereby obtained. Even then, however, the final determination of particulate size is often derived from statistical probabilities, based on various assumptions, which can lead to unreliable results.